Communications networks make use of time division multiple access (TDMA) to share a communication channel among a number of nodes. TDMA is a time sharing technique where different nodes are assigned different segments of time (known as time slots) during which the nodes are permitted to transmit on the communication channel. Assigning non-overlapping time slots helps to avoid collisions between different transmissions. TDMA can be combined with other access techniques, such as code division multiple access (CDMA) and frequency division multiple access (FDMA).
TDMA systems typically include a guard time between time slots assigned to different nodes to accommodate the effects of propagation delay. Propagation delay results because communications signals move at a finite velocity. For example, radio waves move at the speed of light. Accordingly, receivers located closer to a transmitter will receive the communication signal earlier in time than receivers located further away from the transmitter. Guard time helps to accommodate this differential in reception timing caused by propagation delay.
During the guard time, no node transmits over the shared channel. Accordingly, guard time results in lost channel capacity and reduces the efficiency of the communication network. Therefore, it is beneficial to reduce the amount of guard time.
Various attempts to reduce guard time are known in the art. For example, in TDMA cellular systems, mobile stations can be assigned to transmit in an order corresponding to the round trip propagation delay between the mobile stations and the base station, with remote stations furthest from the base station responding first. This approach reduces guard time in a cellular network, because cellular networks are hub-spoke networks. In a hub-spoke network, all transmissions are either originated or received by hub (e.g., the base station), and no spoke to spoke transmissions (e.g., direct mobile to mobile) are permitted.
Unfortunately, some communications systems are mesh networks, where any node can transmit to any other node. Mesh networks present a more complex situation than hub-spoke networks. One technique is known which minimizes guard time in a mesh network by calculating guard times based on the source (transmitting) and destination (receiving) nodes for each message. Obtaining the source and destination information, however, can be complex and cause inefficiency in the network operation. Furthermore, the approach handles situations where transmissions are point to point (one node to one node) and doesn't handle multi-cast (point to multi-point) or broadcast transmissions. The minimum guard time obtained with this technique can also result in negative guard times (e.g., where a later transmission begins before a previous transmission has completed). Negative guard times can be more difficult to handle in actual implementations.